In recent years, research and development have been extensively conducted on light-emitting elements using organic electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting organic compound is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting organic compound can be obtained.
Since such a light-emitting element is of self-light-emitting type, it is considered that the light-emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as flat panel display elements. In addition, it is also a great advantage that the light-emitting element can be manufactured as a thin and lightweight element. Furthermore, very high speed response is also one of the features of such elements.
Furthermore, since such light-emitting elements can be formed in a film form, they make it possible to easily form a large-area element. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements also have great potential as planar light sources applicable to lighting devices and the like.
As described above, application of light-emitting elements using organic EL to light-emitting devices, lighting devices, or the like is expected. On the other hand, there are many issues regarding light-emitting elements using organic EL. One of the issues is a reduction in power consumption. It is important to reduce a drive voltage for the light-emitting element in order to reduce power consumption. Further, the emission intensity of the light-emitting element using organic EL is determined by the amount of electric current flowing therein. Therefore, in order to reduce the drive voltage, it is necessary to feed a large amount of current at low voltage.
Previously, as a method for reducing drive voltage, an approach of providing a buffer layer between an electrode and the layer including a light-emitting organic compound, has been attempted. For example, it is known that a drive voltage can be reduced by providing a buffer layer which includes polyaniline (PANI) doped with camphorsulfonic acid, between indium tin oxide (ITO) and a light-emitting layer (see Non-Patent Document 1, for example). It is explained that this is because of the excellent carrier-injection property of PANI to the light-emitting layer. Note that in Non-Patent Document 1, PANI that is the buffer layer is also considered to be a part of the electrode.
However, as described in Non-Patent Document 1, PANI has a problem that transmittance becomes poor when a film thickness becomes thick. Specifically, it is reported that at a film thickness of about 250 nm, the transmittance is less than 70%. In other words, since the problem is with the transparency of the material itself that is used for the buffer layer, light that is generated within the element cannot be taken out efficiently.
Also, according to Patent Document 1, an approach of serially connecting light-emitting elements (called light-emitting units in Patent Document 1) to improve the luminance per a certain current density, in other words, current efficiency, has been attempted. In Patent Document 1, for a connecting portion of serially connected light-emitting elements, a mixed layer of an organic compound and a metal oxide (specifically, vanadium oxide and rhenium oxide) is used, and it is considered that this layer can inject holes and electrons to light-emitting units.
However, as apparent by looking at an embodiment, for the mixed layer of an organic compound and a metal oxide that is disclosed in Patent Document 1, a high absorption peak is observed not only in the infrared region but also in the visible light region (around 500 nm), and a problem in transparency occurs also. This is due to the effect of an absorption band generated by charge transfer interaction. Therefore, as expected, light that is generated within the element cannot be taken out efficiently, and the light emission efficiency of the element is degraded.